This invention relates to hermetically sealed batteries having glass-to-metal seals and especially to lithium batteries with organic electrolytes containing lithium salts which are intended for long term use in implantable medical devices.
Low silica glasses such as Ta-23 glass were developed for glass-to-metal seals for hermetically sealed batteries with lithium-containing electrolyte compositions. They are highly resistant to lithium ion attack for long term battery applications (more than 2 years of reliable operation and typically 7-10 years of reliable operation) and are highly effective insulators. A feedthrough pin bonded to the center of the glass seal is typically molybdenum due to its resistance to the corrosive effects of lithium battery materials and due to its close match with the thermal expansion characteristics of the glass. However, molybdenum also has a serious disadvantage in that it is difficult to make reliable electrical connections by welding to the molybdenum pin. Molybdenum can react with atmospheric moisture to form an oxide layer which can inhibit effective welding. Molybdenum is also a material with low ductility which makes welding difficult. Other pin materials such as tantalum or niobium, which are known to be more easily welded, would therefore be desirable.
In U.S. Pat. No. 4,556,613 issued to Taylor et al., glass-metal seals for lithium batteries are disclosed with particular applications directed to sulfur dioxide and thionyl chloride cells. Although Taylor et al. does not disclose a glass with a low silica content (such as the silica content of Ta-23 glass) for the feedthrough, it does suggest that such metals as tantalum and niobium are stable with lithium. Similar suggestions are made in U.S. Pat. No. 4,233,372 issued to Bro et al. and in U.S. Pat. No. 4,168,351 issued to Taylor. However, not all of the materials set forth in those patents can be relied on to provide long term corrosion stability in battery feedthrough pin applications. Localized corrosion of metals in the form of crevice corrosion may occur when two metals or nonmetal-to-metal components are joined to form a crevice. These crevices can contain electrolyte which seeps into the interfaces, causing crevice corrosion in the form of pitting attack on certain materials. Feedthroughs prompt concern about corrosion due to the relatively small diameter of the feedthrough pin and the crevices formed between the sealing glass and the pin. Tantalum and other metals have, however, been taught to be suitable for use in feedthroughs outside of battery applications. For example, in U.S. Pat. No. 4,940,858 issued to Taylor et al., a feedthrough for an implantable pulse generator is disclosed in which a feedthrough pin of niobium or tantalum is said to be suitable for use in combination with a fusible glass composition such as Ta-23 glass. However, it is not disclosed that whether these combinations could be suitable to make a corrosion-resistant hermetic seal for lithium batteries.